Keywords:A. Ceramics; B. Chemical synthesis; C. X-ray diffraction; D. Thermodynamic properties
1. Introduction
Self-propagating High Temperature Synthesis is potentially an energy-efficient pro-cess to synthesize many inorganic materials, including intermetallics, ceramics, andceramic composites [1–3]. Characteristics of the process in the combustion-wave modeare self-generated high temperature (800 to 3500oC), relatively rapid propagating com-bustion fronts (0.1 to 10cm/sec), high rates of heating (up to 106 deg/sec), and thermalgradients (up to 107 deg/cm) at the combustion front. The exact values of temperature,
wave velocity, thermal gradients, and rate of heating are functions of the particularchemical system and experimental parameters. Both solid-solid and gas-solid combus-tion reactions are used to produce a variety of advanced technological materials.
Al2O3-TiC, Al2O3-ZrO2, Si3N4 and TiC which have high temperature strength, highthermal shock resistance have used as advanced structural materials. Especially, the TiC-Al2O3 composite used for making abrasive tool and wear resistant coating to protectcomponents of oil refining equipment has been produced by hot pressing TiC and Al2O3
powder. The preparation of TiC-Al2O3 composite powder by the SHS process was widelystudied in several former researches [4–7], however the effect of carbon sources on thecombustion characteristics are rare. Hence, in this study, the SHS process was applied to asystem of TiO2/Al/C for the production of TiC-Al2O3 composite. The reaction in this systemwas discussed in terms of the mixture ratio and the preheating of the pellet for the formationof fine TiC-Al2O3 powder when activated charcoal, carbon black and graphite used as carbonsources.
2. Experimental
The raw materials used were TiO2 (98.56% rutile,,0.5mm; Korea Titanium Co.,Korea) and Al (99.5%,,44mm; Chang Seong Co., Korea). Activated charcoal(DuksanPharm. Co., Japan), Carbon black(LG Chemical Ltd., Korea) and Graphite (KantoChemical Co., Inc., Japan were used as carbon sources. The reactants(TiO2:Al:C53.0:3.4;4.6:1.8;3.6), were mixed by milling for 5 hours using an alumina ball mill. Themixed powders were pressed to form a pellet 40mm diameter and 50;60mm in heightunder a pressure of 80MPa and then dried in a drying oven of 70oC for 1 hour. The greenpellet was ignited in a reactor using a tungsten coil connected to a power supply underatmosphere of argon. The green pellet was preheated by small furnace installed incombustion chamber from room temperature to 600oC. The combustion temperature, thecombustion velocity and the cooling rates of the pellet were measured by a c-type(W-5%Re vs. W-26%Re) thermocouples embedded into the pellet using a data acqui-sition system and a personal computer. The crystal structures of reaction products were
Table 1Thermochemial data for TiC, Al2O3, TiO2, Al and C [9,10]
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Fig. 1. X-ray diffraction patterns of reaction products varying with carbon molar ratio at TiO2: Al53.0:4.0(carbonsource:charcoal activated):(a)1.8 (b) 2.1 (c) 2.4 (d) 2.7 (e) 3.0 (f) 3.3 (g) 3.6.
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Fig. 2. X-ray diffraction patterns of reaction products varying with aluminum molar ratio at TiO2: C53.0:3.0(carbon source: charcoal activated): (a) 3.4 (b) 3.6 (c) 3.8 (d) 4.0 (e) 4.2 (f) 4.6.
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analyzed using X-ray powder diffraction (XRD) and the microstructure was investigatedusing scanning electron microscopy (SEM).
3. Results and discussion
The adiabatic temperature (Tad) can be used as a general indication of the temperature atthe combustion front. It can also be used in a semiquantitative way to ascertain whether thesynthesis of a given material can be accomplished by a self-propagating method. It has beenempirically suggested that combustion reactions will not become self-sustaining unless Tad
$ 1800K [8]. The reaction of this experiment can be represented as follows:
3TiO2 1 4Al 1 3C3 3TiC 1 2 Al2O3 (1)
The theoretical adiabatic temperature of this reaction can be calculated using thermody-namics data in Table 1 as follows;
Fig. 3. The effect of the preheating temperature on the combustion temperature.
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Q 5 (2DH2980 ) 5 E
298
Tad
OCp~products!dT (2)
DH2980 5 ~3DH298,TiC
0 1 2DH298,Al2O3
0 ! 2 ~3DH298,TiO2
0 1 4DH298,Al0 1 3DH298,C
0 !
5 2 1071.58kJ/mol
(2DH2980 ) 5 E
298
Tad
OCp~products!dT
Finally, 10715805 E298
1800
~3Cp,TiC~s! 1 2Cp,Al2O3~s!!dT
Fig. 4. The effect of the preheating temperature on the combustion velocity.
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1 E1800
2327
~3Cp,TiC~s! 1 2Cp,Al2O3~s!!dT
1 DHm,Al2O3E2327
Tad
~3Cp,TiC~s! 1 2Cp,Al2O3~l!!dT
[Tad < 2546K
Therefore, we can assume that the SHS reaction of Eq. [1] will be possible from thepreceding theoretical result. Actually, the combustion temperature, Tc, measured in thissystem was about 1700;2250K because of heat losses.
The effect of the carbon molar ratio on the XRD of product without preheating sample isrepresented in Fig. 1. The molar ratio of carbon was changed from 1.8 to 3.6mole, becausethe value of x in TiCx compound is in the range of 0.6;1.0, then the content of the total
Fig. 5. Arrhenius plot of Al2O3-TiC combustion reaction varying with carbon source.
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carbon in the product should be between 1.8 and 3.0 mole from eq. [1]. As the carbon contentincreased, the unreacted Ti diminished and was completely eliminated when the carboncontent was over 2.7 mole shown in Fig. 1. Further, the XRD patterns in Fig. 1 now showthe presence of carbon when the value of C in the ratio TiO2:Al:C exceeds 2.7. Fig. 2 showsthe X-ray diffraction patterns of the products with various Al molar ratios. As the Al molarratio increases, the amount of unreduced TiO2 is seen to decrease until the Al:TiO2 reachesa ratio of 4.0:3.0 when all the TiO2 is completely reduced. Peaks of Al appear above that ratiobecause of an excess of Al over stoichiometry.
Figures 3 and 4 show the effect of the preheating temperature on the combustiontemperature and the combustion velocity of the reactions varying with carbon source. Asexpected, the combustion temperature and the combustion velocity of the reaction increased
Fig. 6. X-ray diffraction patterns of reaction products varying with carbon source(TiO2: Al: C53.0:4.0:3.0); (a)charcoal activated (b) carbon black (c) graphite.
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Fig. 7. SEM photomicrographs of synthesized products varying with carbon source: (a) charcoal activated (b)carbon black (c) graphite.
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as the preheating temperature increased. According to these figures, charcoal as carbonsource has the highest combustion temperature and velocity of all. In room temperature, thecombustion velocity was 0.5;1.5mm/sec varying with carbon source. When the initialtemperature of reactants is 600oC, the combustion velocity of charcoal activated, carbonblack and graphite were 14.2mm/sec, 8.65mm/sec and 6.06mm/sec respectively. Fig. 5shows the activation energy which is calculated using measured combustion temperature andvelocity on the combustion reaction. It shows that activated charcoal, graphite and carbonblack values are 308kJ/mol, 316kJ/mol and 345kJ/mol respectively.
Figures 6 and 7 show that the X-ray diffraction patterns and the microstructure of theproduct vary with the carbon source at TiO2:Al:C53.0:4.0:3.0 mole. These figures showthat crystal structure and microstructure of the product was not affected by carbonsources.
In this work, we used a wedge type copper mold to investigate microstructure of reactionproduct as to the different cooling rates. The cooling rate is changed according to thediameter of the wedge by the large heat conductivity of copper. Fig. 8 shows the temperature
Fig. 8. SEM photographs of synthesized products with various cooling rate.
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profile and microstructure at different mold positions (TiO2:Al:C53.0:4.0:3.0, activatedcharcoal without preheating). As the cooling rate become faster, the grain size becomesmaller. We can see the result that the Al2O3 particles grew bigger and TiC particles seemto be sintered in Fig. 8(a), the slowest cooling rate. In the case of fastest cooling rate, Fig.8(d), the grain size of alumina becomes smaller (up to 3;4mm) and the grain size of TiC isabout 2mm independently. So, the grain size of product depends on the cooling rate after thecombustion reaction.
Chemical Technology and Metallurgy, Chernogolovka, 1975.[9] I. Barin, Thermochemical Data of Pure Substances, (Eds.) H.F. Ebel, C. Dyllick-Brenzinger, VCH (1989).
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